1. Field of the Invention
This invention relates to a process of molding a polyurea based composition, a polyurea based RIM composition, and molded articles obtained thereby by RIM and spray molding.
2. Discussion of the Background
Polyurethane polymers prepared from an active hydrogen-containing compound and an isocyanate are widely employed in molding processes, particularly reaction injection molding (hereinafter RIM) processes. RIM articles are finding increased usage as automotive fascia.
The basic polyurethane polymer systems are typically based on an OH polyol component, an OH cross-linker and an isocyanate component. However this system suffers from long cream, demolding and cycle times, greatly increasing the processing time. Modifications to the basic polyurethane system to shorten these processing times has been achieved through substitution of the OH cross-linker with an aminic cross-linking system. Typically, thermosetting urethane polymer compositions comprise an isocyanate component with an excess of isocyanate groups and an aromatic diamine as a chain extender, to form a polyurea-type urethane polymer. Optionally, the polymer composition may also contain additional amounts of a reactive polyol to form a hybrid urea-urethane polymer. Such systems greatly decrease the cream and demolding times, therefore enabling much shorter cycling times in a RIM process.
The use of chain extenders, such as di-alkyl aromatic diamines, and more particularly di-ethyltoluene diamines and di(alkylthio)aromatic diamines are often used with isocyanate pre-polymers alone or with a polyol component to form a polyurea/polyurethane molding RIM composition (for example U.S. Pat. No. 4,595,742, 4,631,298, and 4,786,656). While an increase in the flexural modulus is observed through the addition of di-alkyl(thio)aromatic diamines, these compositions are still limited with respect to flexural modulus without observing "cold break" on demolding. "Cold Break" is a brittleness observed in the molded article during demolding. The presence of cold break causes the molded article to fracture on demolding. When trying to achieve a higher flexural modulus, to above 80,000 psi, by increasing the isocyanate content of the isocyanate component (i.e. higher % NCO), these materials suffer "cold break". Alternatively attempts to increase the flexural modulus by increasing the functionality of the polyol component also suffers from "cold break". Accordingly, efforts to increase the flexural modulus while preventing "cold break" is an on going area of research.
In addition to the mechanical properties of the polyurethane polymer, the processing of the polymer systems plays an important role in the usefulness of a polyurethane system. In RIM processing, a short gel time is desired to increase the productivity of the overall process. However, polyurethane systems based on an OH polyol component, an OH cross-linker and an isocyanate component had a gel time of from 5-8 seconds and a cycle time of from 3-3.5 min. Polyurethane systems using -NH.sub.2 crosslinkers and -OH polyols (for example U.S. Pat. Nos. 4,595,742, 4,631,298, and 4,786,656) reduced the gel time to about 1.2 seconds and the cycle time to 1.5-2 min. This greatly increased the productivity of RIM processes using these systems, but these -NH.sub.2 crosslinkers and -OH polyols systems suffered from an inability to increase the flexural modulus above 80,000 psi without observing "cold break" without the addition of fillers.
Polyurea systems based on amine terminated polyether resins and aminic cross-linkers have been developed (U.S. Pat. No. 4,433,067 and 4,444,910), which afford superior heat resistance and mechanical properties, and due to the extremely high reactivity of the amine terminated polyether resin, the gel times are extremely short, in the range of 0.7 seconds. Such a rapid reaction rate makes these systems very difficult to manipulate, and also severely limits the type of RIM technique for which such a composition is suitable. For example, Structural Reaction Injection Molding (S-RIM) is not applicable to such a system due to the short reaction time, resulting in incomplete wetting of the structural reinforcement.
It has also been discovered that the flexural modulus of a polyurethane composition based on MDI-polyol prepolymer and an aromatic diamine can be increased beyond 80,000 psi at 75.degree. F. while being substantially free of "cold break" by the addition of a "cold break" additive such as triethanol amine or a triethanol amine-ethylene oxide end capped reaction product (U.S. Pat. No. 5,059,634). However the mechanical properties and processability of the "cold break" containing polyurethane compositions can still be improved.
The polyether polyol component in a polyurethane system is typically based on ethylene oxide or propylene oxide or a mixture of the two. Block copolymers may be manufactured by first reacting propylene glycol with propylene oxide to form a homopolymer followed by reaction with ethylene oxide to give a block copolymer. Since the primary hydroxyl groups resulting from ethylene oxide are more reactive than secondary hydroxyl groups, the polyols produced in this manner are more reactive. In this fashion "EO capped" polyether polyols are produced. Random copolymers are obtained by polymerizing mixtures of propylene oxide and ethylene oxide.
However, in RIM polyurethane systems, the amount of ethylene oxide is generally limited to less than about 30 wt. %, either as "EO capping" or as part of the random copolymer. High amounts of ethylene oxide was thought to produce a polyol component with inferior mechanical properties and hygroscopicity in the resulting polyurethane.
In addition, there exists a need to improve existing fiberglass spray molding processes. Typically, these systems are based on polyester resin which are spray molded to form automotive body panels. However the polyester resin systems have an environmental drawback in the release of large amounts of volatile organic compounds, particularly styrene. The Clean Air Act of 1990 (Title 42, section 7412(b)) has identified styrene as a hazardous air pollutant. Accordingly, it is anticipated that new government standards will be issued to limit the release of styrene into the environment, thereby greatly increasing the cost associated with traditional fiberglass spray molding processes. A spray molding process with low volatile organic compound emissions is therefore desired.
Consequently, research continues into systems with excellent mechanical properties, but more flexible gel times and the elimination of "cold break" and which are simple to make.